Which Firewall rules need to be adjusted?

[Nic2012]

Hi,
we have implemented Java Push feature in our app server JBoss-EAP 7.3, Java 11. There is a customer whose users cannot receive push notifications in the Chrome browser. Although each user has successfully subscribed push and in this case we can store the endpoints. Later during runtime when server side code decides to push notification on each endpoint, users complain about that notifications are not received on their workstations (Win10, Chrome latest).

But testing push generally via https://gauntface.github.io/simple-push-demo/ is working for those users, so it cannot be caused by incoming connection restrictions on client machines.

According to
https://firebase.google.com/docs/cloud-messaging/concept-options#messaging-ports-and-your-firewall

which outgoing connection rules - domain, hostnames or ports - may be to adjusted on server side network or would you recommend?
Unfortunately your API description mentioned no details about possible firewall restrictions. In the code I cannot find any evidences which ports are used when sending push notifications to the push providers.

Hopefully you can clarify this issue. Thanks and best regards,
Nic